How to slide one object on another, keeping curvature?
If I were you I would probably create a curve that matches the curve of the character, then use a curve modifier on the glasses.



If you don't want the goggles to distort, then you could parent the goggles to the path then you can move along the path by changing the frame (left/right arrows on your keyboard). That doesn't work if you are also animating the character though.